The invention is in the field of optics, specifically in using the resonance of optical waveguide micro-resonators as chemical, biological or physical sensors.
Optical sensors can be broadly categorized, in decreasing order of size into free spaced sensors, fiber based sensors and integrated optics sensors. The main advantages of integrated optics sensors are that it is possible to fabricate them using the well established traditional silicon integrated processing, and that is possible to make very small but high functionality devices with low cost. There are, however, difficulties with the integrated optics approach, the primary one being that current devices are large and packaging the non-integrated components such as lasers can be expensive.
There have been many implementations of integrated optical waveguide sensors. However, these prior art has focused primarily on straight waveguides with using a differential TE/TM measurement on waveguides. Currently, the most sensitive integrated optics sensors use a differential TE/TM method. The principle idea is to use a waveguide that is coated with a thin layer of material and that has a strong affinity for the substance to be detected. If the material to be detected is present, it will stick to the binding layer of the material and the difference in interaction of the evanescent fields of the TE and TM modes can be measured as a difference in phase, by interfering the TE and TM modes together at the output facet.
In addition, the waveguides that have been used have primarily been slab waveguides, since this differential TE/TM method is so sensitive that scattering due to sidewall roughness would result in a severe reduction in efficiency. These TE/TM differential methods also suffer from the fact that a long interaction is needed and the lack of horizontal confinement in the slab waveguides. These two facts make these methods large and difficult to integrate on a large scale. Another drawback of this method is the need for a stabilization scheme to prevent the sensor from being completely dominated by environmental changes.
The invention provides an optical micro-resonator used as a sensor. Using a laser and a detector it is possible to detect changes in the position of the resonance position, which would in turn be dependent on the chemicals, which have been adsorbed by chemically or biologically sensitive material.
Integrated optical difference interferometers have been widely studied in the literature. These devices take advantage of the difference in interaction between the TE and TM modes and a chemical or biological agent on the surface of the waveguide. Using a micro-resonator with a TE and a TM mode, this same effect can be applied to a micro-resonator. With a high Q (xcx9c10000) resonator the effective path length can be made to be the order of 1 cm, and the sensitivities are very high. The invention utilizes optical waveguide micro-cavity resonators as sensors and further methods to improve the usability and cost. In addition, a control resonator can be fabricated close by and any environmental changes can be monitored. Resonators make very good sensors. For example, small amounts of chemicals or biological compounds bound to the surface of a micro-cavity can lead to changes in the effective index which in turn leads to large changes in the position of a resonance peak which may be easily observed. Thus, these resonators can be used as very sensitive sensors. Scanning a device for a resonance requires either a white light source with a spectrometer or a wavelength tunable source, both of which are expensive. In accordance with the invention, dithering the position of the resonance and using a single wavelength narrow band laser source to excite the wavelength cavity is explored. When the resonance of the resonator overlaps with the wavelength of emission, the output power that can be measured by a detector will drop. If there is an external change to the resonator due to the change in what is being sensed, the resulting peak shift will be overlapped with the dithering of the resonance position. This may be detected using electronics. The dithering of the resonator can be based on thermo-optic, electro-optic or some other means.
Furthermore, all the techniques, which have been developed from the slab waveguide differential TE/TM sensors, can be easily applied to micro-resonator sensors by simply studying the difference in position of the TE and TM resonances. Thus, when chemicals or biological compounds bind to the surface of the resonator, the TE and TM modes will respond differently. This leads to an effect index change that can be picked up as shifts in the resonance line spectrum.
One of the drawbacks of the resonance technique is that the spectral response is required. The implication being that a tunable laser or a spectrometer is required. However, this is very expensive and difficult to integrate. Furthermore, tunable lasers and spectrometers with large spectral ranges are usually not robust. Hence, it is essential to come up with a scheme to eliminate this problem. The invention provides a resonance dithering method, in which, the resonator line position is changed using an external tuning mechanism like thermal tuning. Such a method would allow the spectrum to be obtained without a tunable laser or spectrometer.